The invention concerns processes for processing the surfaces of a container, of thermoplastic material, for example.
By way of example, the invention will find application in the field of depositing thin barrier-effect layers on bottles or jars made of thermoplastic materials such as polyethylene terephtalate.
Indeed, there is currently a search to improve the barrier properties of these containers, particularly with a view to decreasing their permeability to gas or to increase their opacity to certain radiations, such as ultraviolet, in order to increase the preservation time of the products packaged in these containers.
To that end, various processes have been proposed that seek either to modify the material of the container directly, at least at its surface, or to coat the container with a layer of organic or inorganic material in order to improve the container's properties. One particularly favorable way to achieve this consists of low-pressure plasma processing. In such a process, a vacuum is created inside the enclosure at the same time as a reaction fluid is injected into it under an absolute pressure preferably of less than 1 mbar. The reaction fluid varies, depending on the nature of the material to be deposited. It includes a precursor of the material to be deposited, generally in the form of a gas or a gas mixture. It can also include a carrier gas.
This reaction fluid is subjected to microwave-type electromagnetic radiation suitable for exciting the precursor in order to form a plasma that creates active molecules. In the case of processing by deposit, these molecules can be deposited on the surface of the container with a particularly strong physical-chemical bond that guarantees the stability of the deposited material. Nevertheless, in some cases, the processing can consist of a simple modification of the surface of the material of which the container is made. There is then no deposit of a new layer of material, but rather a modification of the containers material by interaction with the molecules or active parts of the plasma.
In particular, the use of microwave type electromagnetic radiation makes it possible to obtain deposits having a special structure that is impossible to obtain with other types of radiation, such as widely used radio frequency radiation.
One of the difficulties encountered in implementing these processes is in obtaining a uniformity of the processing over the whole surface to be coated. In the case of processing by deposit, these problems of homogeneity can have repercussions in terms of thickness of the deposited layer and in terms of the composition of this layer. Obviously, this poor homogeneity of the deposited layer is not satisfactory.
To obtain a homogeneous treatment, then, the use of a plasma offers the greatest possible uniformity.